Particulate matter (PM), also known as particulates, are tiny pieces of solid or liquid matter in air. Such particulate matter may comprise bacteria, mold spores, pollen, house dust mite allergens, viruses, dust, smog, tobacco smoke, etc. Inhaling of particulate matter has shown to cause negative health effects on humans and animals and major health benefits can be achieved by reducing particulate matter in the air. Air filter arrangements are used in a wide variety of environments to remove particulate matter in order to purify air. Though there exist several types of air-filtration technologies, electrostatic filters have become increasingly popular because of their high efficiency. For example, the document US 20070137486 A1 describes an electrostatic filter.
Further, the document US 2011/0219954 A1 relates to an electrically enhanced air filtration system which uses rear fiber charging. In particular, a fibrous filter may be placed in an upstream position within the system with one or more ionization arrays positioned downstream or to the rear of the fibrous filter in terms of airflow direction. The fibrous filter may include a grounded side and fiber side with the grounded side being upstream of the fiber side. The ionization array may include a plurality of electrodes each extending unidirectionally toward the fibrous filter. The document GB 2308320 A relates to an air filtration apparatus comprising an inlet, an outlet, a fan, a corona discharge cell and an electret filter e.g. polypropylene. In another embodiment, two discharge cells are provided in sequence, and two electret filters the first being thicker than the second. The return current from the filter element can be measured to give an indication of levels of pollutant particles present, the indication being used to control the speed of the fan and show when the filter needs to be replaced. The document WO 9609118 A1 relates to an electro-mechanical electrostatic air cleaner that combines a low air resistance dielectric fibrous filter material such as polyester or glass which is positioned between and electrically charged by two electrically resistant carbon coated screens encased in an electrically insulated frame. The screens are charged by a remotely mounted bi-polar power supply.
An electrostatic filter works with the principle that portions of the electrostatic filter having a certain electrostatic charge attract particles in the air. Thereby, an electrostatic filter may have a higher particulate capturing capability than an ordinary filter without portions having an electrostatic charge. However, the electrostatic charge of an electrostatic filter may decline over time, for several reasons. Thus, also a particulate capturing capability of such an electrostatic filter may decline over time. Further, some air filtering apparatuses utilize filters having relative dense filter media. Due to the relative dense filter media, such filters will have a relatively high flow resistance. As a result, a considerable amount of electrical energy will be required to drive a fan forcing ambient air through such a filter. Also, it is likely that such an arrangement will produce a considerable amount of noise.
Accordingly, in view of the prior art arrangements, there is a need for an improved air filter arrangement.